Java for LeetCode 144 Binary Tree Preorder Traversal

標籤：

Given a binary tree, return the preorder traversal of its nodes‘ values.

For example:
Given binary tree {1,#,2,3},

   1         2    /   3

return [1,2,3].

二叉樹的前序走訪，根節點→左子樹→右子樹

解題思路一：

遞迴實現，JAVA實現如下：

    public List<Integer> preorderTraversal(TreeNode root) {List<Integer> list = new ArrayList<Integer>();if (root == null)return list;list.add(root.val);list.addAll(preorderTraversal(root.left));list.addAll(preorderTraversal(root.right));return list;    }

 解題思路二：

使用stack實現，JAVA實現如下：

public List<Integer> preorderTraversal(TreeNode root) {List<Integer> list = new ArrayList<Integer>();if (root == null)return list;Stack<TreeNode> stack = new Stack<TreeNode>();stack.push(root);TreeNode pop = root;while (!stack.isEmpty()) {pop = stack.pop();list.add(pop.val);if (pop.right != null)stack.add(pop.right);if (pop.left != null)stack.add(pop.left);}return list;}

 

Java for LeetCode 144 Binary Tree Preorder Traversal